For the purpose of preventing air pollution, an evaporation system (evaporated fuel processing system) for processing evaporated gas is mounted to an automobile to prevent the evaporated gas (evaporated fuel) generated in a fuel tank from being released into the atmosphere.
In North America, it is further prescribed by law to check whether or not an abnormality such as breakage, a crack, or a hole is present in piping itself of the evaporation system at a specific frequency. Accordingly, it is necessary to perform a leak diagnosis with the piping of the evaporation system and the following method is generally used: a pressure is applied to the piping of the evaporation system in a sealed state, and a fluctuation of the pressure is monitored.
The method in which the pressure is applied to the piping of the evaporation system during the leak diagnosis differs depending on an automobile manufacturer, and the method includes a method that uses an air pump and a method that uses a pressure fluctuation caused by natural heat radiation.
While the leak diagnosis in the case of using the natural heat radiation is capable of performing the diagnosis without generating a noise, there are disadvantages that a long diagnosis time increases total power consumption and also that a sufficient diagnosis frequency cannot be secured.
On the other hand, the leak diagnosis in the case of using the air pump can be carried out in a short time period to thereby achieve power saving and secure the sufficient diagnosis frequency. However, as described about a configuration for absorbing an operation sound of a leak diagnosis apparatus in Patent Document 1, and a configuration for alleviating a collision inside a leak diagnosis apparatus in Patent Document 2, there occurs a problem that a noise is generated during the diagnosis. Since the leak diagnosis in the case of using the air pump is performed under a quiet condition during stop of an engine, the noise during the diagnosis, in particular, poses a problem as an abnormal noise.
Note that Non-Patent Document 1 describes a general sound absorbing structure that absorbs sound, but is not limited to the noise generated during the leak diagnosis.